In recent years, a post-finishing apparatus, which performs post-finishing operations, such as stitching, folding, and adhering processes, after sheets carrying formed images are aligned on a stacker, is connected to an image forming apparatus, such as an electro-photographic copying machine, a printer, a facsimile and a compound machine which compounds these, whereby an image forming system is structured. Due to this, the operator is saved the trouble of stitching the sheets, carrying the formed image by hand, and thereby the operator more efficiently conducts office duties.
In such post-finishing apparatus, after each sheet, which has curled during previous procedures, is placed on a sheet table, without alignment or at an angle, the post-finishing operations are conducted to the sheets. Therefore the sheets are not aligned, and their appearance is worse, which is a problem of overall quality.
In the conventional art, to overcome this problem, the sheets are struck against an alignment member which is placed perpendicular to the conveyance direction of the sheet, and further, manufacturing accuracy of conveyance members has been improved, thereby displacement and angled placement are corrected.
As an example which overcomes this problem, Unexamined Japanese Patent Application Publication No. 11-199,121 discloses a sheet processing apparatus in which plural drawing paddles are provided to adjust the tops of the sheets (see four positions shown in FIG. 6).
In the sheet processing apparatus of the above patent example, since the drawing paddles are fixed at predetermined positions, the various portions of the drawing paddles come into contact with the sides of various sheets, that is, the contacting portions depend upon the size of the sheet. Therefore, the effects of the paddle differ based on the size of sheet. Specifically, when both sides of the sheet are largely curled, the touched portions of the drawing paddles shift inside in the case of particular size of sheets. Accordingly, the curl, which is out of the touched portion of the drawing paddles, still exists, that is, though the alignment member is used, the sheets are stacked without being aligned, which does not overcome irregularity of the stacked sheets.
Further, the shape of the curled sheets differs, based on the type of sheets, which may be convex or concave curled. For example, the shape varies depending on whether it is a recycled sheet, a thin sheet, a thick sheet, a high quality sheet, a flipped sheet, and a sheet carrying a color-toner image.